Dissolved organic carbon bioreactivity and DOC:DIN stoichiometry control ammonium uptake in an intermittent Mediterranean stream

Heterotrophic organisms in streams use dissolved organic carbon (DOC) and dissolved inorganic nitrogen (DIN) from the water column to meet their growth and energy requirements. However, the role of DOC availability in driving DIN uptake in headwater streams is still poorly understood. In this study, we focus on how DOC:DIN stoichiometry and DOC bioreactivity control ammonium (NH4+) uptake and heterotrophic aerobic respiration, and how this influence varies among seasons in a forested Mediterranean headwater stream. We estimated in-stream NH4+ uptake rates seasonally by conducting whole-reach constant-rate additions of NH4+ with and without amendments of either lignin (recalcitrant DOC) or acetate (labile DOC). During each addition, we characterised microbial community composition by molecular analyses, stream metabolism with the single-station method, and heterotrophic aerobic respiration by adding a metabolic tracer (resazurin). The stream was heterotrophic (net ecosystem production <0) regardless of the season, with a microbial community mostly composed of heterotrophic bacteria. In-stream NH4+ uptake rates were not related to either background NH4+ or DOC concentrations. Instead, these rates increased with increasing the molar ratio of NH4+ to nitrate (NO3-) (NH4+:NO3-) and DOC to DIN (DOC:DIN).Whole-reach heterotrophic aerobic respiration rates showed the same relationship against stoichiometric ratios as NH4+ uptake rates. Furthermore, in-stream NH4+ uptake rates were from 5% to >800% higher during the co-additions of acetate than when adding NH4+ either alone or with lignin. Our results indicate that in-stream NH4+ uptake was largely controlled by heterotrophic bacteria, and that the stoichiometric balance between organic resources and nutrients was key to explaining the variability of in-stream NH4+ uptake and heterotrophic aerobic respiration. Moreover, the observed increase in NH4+ uptake during acetate additions suggests that heterotrophic activity was limited by labile DOC availability. Our study highlights that both DOC:DIN stoichiometry and DOC bioreactivity are relevant factors driving the seasonal pattern of in-stream N processing in this forested Mediterranean headwater stream.